1. Field of the Invention
The present invention relates to earth boring bits of the type used to drill oil and gas wells.
2. Description of the Prior Art
The prior art earth boring bits include the rolling cutter bits, having either steel teeth or tungsten carbide inserts, and diamond bits which utilize either natural diamonds or artificial or man-made diamonds. The diamond earth boring bits have one-piece bodies of either steel or matrix. The steel body bits are machined from a steel block and typically have cutting elements which are press-fit into recesses provided in the bit face. The matrix bit is formed by coating a hollow tubular steel mandrel in a casting mold with metal bonded hard material, such as tungsten carbide. The casting mold is of a configuration which will give the bit a desired form. The cutting elements are typically either polycrystalline diamond compact cutters braised within a recess provided in the matrix backing or are thermally stable polycrystalline diamond or natural diamond cutters which are cast within recesses provided in the matrix backing.
The single-piece bit is finding increased applications in both directional drilling and the drilling of sticky formations. One problem encountered in designing a one-piece bit is that the gage region, i.e. the uppermost end as viewed during drilling, exhibits a great deal of wear in directional and high speed drilling applications. The prior art techniques for design of the gage region of the one-piece bit have typically been of two schools of thought. The first has been to build full API gage sections with a high percentage of contact with the borehole wall. The second approach has been to undersize the upper gage region to limit wall contact in sticky formations. Neither of these techniques has proved entirely satisfactory. Full gage sections can create torquing problems, sticking and less than optimum directional characteristics. Short, full gage sections backed up by undersized lands can go undersize, cause deviation and allow unacceptable bit wobble. If unchecked, the rapid wear of the gage region and resulting wobble of the bit can cause the cutting structures to wear prematurely, limiting the useful life of the bit.
An object of the present invention is to increase the effective gage length of a single-piece bit without increasing the total wall contact area.
Another object of the invention is to provide a bit with an effective gage length having cutting elements at the top and bottom of the gage with an intermediate undercut region which reduces drag and improves the stability of the bit.
Another object of the invention is to provide a single-piece bit with changeable directional characteristics.
Additional objects, features and advantages will be apparent in the written description which follows.